1. Field of the Invention
This invention relates to liquid treatment method, and more particularly is directed to improvements in upflow ion exchange and filtering apparatus filters, i.e., apparatus in which a liquid to be treated is passed upwardly through a filter bed of granular solids.
2. Description of the Prior Art
Upflow filters and ion exchange equipment have been in use for a number of years. Examples of such units are shown in U.S. Pat. No. 1,891,061, issued 1932; U.S. Pat. No. 2,855,364, issued 1958; and U.S. Pat. No. 3,202,286, issued 1965. It has long been known that filtering or treating liquid which is to be passed through a bed of granular solids is more efficient if the liquid is flowed upwardly through the bed instead of downwardly, because the solids are automatically distributed with the coarser ones at the bottom and the smaller ones at the top of the bed. However, upward flow involves the problem of the bed lifting, cracking, or fluidizing, with the result that the liquid passes freely through the bed without undergoing the filtering or treating action intended by the granular solids. The disruption of the bed is particularly a problem when it is desired to flow the liquid through the bed at a relatively high rate. This problem has been partially solved by the use of a grate in contact with the upper portion of the bed to prevent it from rising or becoming fluidized under relatively high throughput rates. Typical examples of grates are shown in U.S. Pat. Nos. 2,855,364 and 3,202,286 referred to above.
However, even with a grate, the bed eventually becomes so loaded with trapped material that it must be cleaned. This is typically done by increasing the flow of liquid through the bed, and sometimes by also adding air, to cause the bed to break up and permit the trapped material to be released and flushed out as liquid flows up through the fluidized bed. After the granular solids are cleaned of trapped material, the flow of liquid up through the filter is reduced or stopped, and the granular solids are permitted to settle back into a static bed. The settling results in a natural classification of the granular solids, with the larger, heavier solids settling to the bottom of the filter bed, and the lighter or smaller particles progressively coming to rest at higher elevations in the bed. This provides an effective use of the bed, especially when particulate material of different sizes is to be removed from liquid undergoing filtration. The coarser solids at the bottom of the bed take out the larger particulate material so that it does not reach the zones of the bed where the finer particles are more densely packed, leaving these upper sections of the bed to do a more effective job on the smaller particulate material which flows past the coarser solids in the bed.
Although the grate referred to above extends the treatment cycle by restraining the bed in a static condition as the pressure drop across the bed increases, the grate presents a problem at the end of the washing cycle when the granular solids start to settle back to their static position. Depending on the specific gravity of the granular solids and the liquid being treated, as well as the viscosity of the liquid, the granular solids sometimes tend to settle on the grate and not pass down through it to provide proper packing density for acceptable liquid treatment. This is particularly true if the grate openings are made relatively small to provide maximum retention of the filter bed during the treatment cycle. Small grate openings also make it more difficult to disrupt the bed when it is time to start the wash cycle.